Embodiments of the present invention relate to power supply for machinery rotating at high speed in general, and relate specifically to an electrical gear for subsea machinery to be run at high rotational speed. More precisely, embodiments of the present invention relate to an electrical gear that effects transformation of AC-voltage and frequency of the electrical power supplied to machinery that is to be rotated at high speed. In analogy herewith, embodiments of the present invention also relate to a method for operating a high speed rotating machinery. Embodiments of the present invention are particularly useful for power supply to electric motors in subsea pumps or compressors located at long offset distances from an offshore platform or land based facility.
Asynchronous or synchronous electrical motors are typically supplied alternating current (AC) at an oscillation rate or frequency that provides the required rotational speed based on the number of magnet poles in the motor. Since at least two poles are required to generate rotation, the motor speed is the product of 120 and the voltage frequency in Hz (oscillations per second), divided by the number of poles 2, 4 or 6, etc. A voltage frequency of 300 Hz is thus required if a motor speed of 9,000 rpm is desired from a 4-pole motor. As for the asynchronous motor the speed will be slightly slower due to slip while the synchronous motor provides full torsional effect as it runs in synchronization with the alternating magnetic field, and thus maintains its rotational speed also under load, up to a maximum load.
In subsea operations, long distance transmission cables are often required to supply electrical power from a sea- or land-based platform to pumps and compressors located subsea. However, long distance transmission of AC-power at voltages and frequencies adapted for fast running synchronous motors demands for extraordinary and costly measures to avoid transmission losses in voltage and power. A method for avoiding high power losses in subsea operation is to implement a complex subsea Variable Frequency Drive (VFD) at the cable end close to the machinery. The VFD rectifies the incoming 3-phase power and modulates a high frequency output power by using controlled power electronics semiconductor devices such as insulated gate bipolar transistors (IGBT), e.g. The VFD will also comprise an inlet transformer for voltage step-down.
High voltage alternating current at frequencies of 50 or 60 Hz is typically available on the platforms (where they also can be converted to lower frequencies by implementation of a platform VFD). Thus in order to avoid losses resulting from long distance transmission of electrical power in subsea operation, electrical power is regularly supplied at a voltage and frequency that require transformation and regulation to fit the specification of an electrical motor running at high speed. As used in this specification, the term “high voltage” refers to a voltage of 10,000 V and above, whereas “medium voltage” refers to a voltage below 10,000 V.